The traditional approach to speed limit enforcement involves police officers observing the traffic and then chasing, stopping, and issuing citations to drivers who speed. The more modern approach to speed enforcement involves the use of a speed measuring device for measuring vehicle speeds and triggering a high speed camera to take photographs of speeding vehicles. The recorded photographs are then used to identify the registered vehicle owners or actual drivers of offending vehicles, to whom citations would be issued. Regardless of the approach adopted, the ability to provide accurate vehicle speed measurements is crucial for any speed law enforcement device. A number of methods exist for measuring vehicle speeds.
VASCAR, which stands for “visual average speed computer and recorder,” is an average speed calculator that measures and displays vehicle speed based on the time a target vehicle takes to travel a known distance. Due to its semi-manual operation nature, the accuracy of VASCAR is subject to human errors and is often challenged in court.
Inductive loop sensors are commonly used to detect the presence/absence of a vehicle based on the inductance changes caused by movements of a vehicle in the vicinity of the loop sensors. There are several systems that attempt to measure the speed of a vehicle by analyzing the waveforms of the output signal as the detected vehicle passes the sensor. One simple method is to use two inductive loops and calculate vehicle speeds by measuring the distance between the two loops and the difference in the time of detection of the vehicle at the two loops. For inductive loops to work, the sensors have to be placed beneath the road surface. Therefore the requirement to close lanes of traffic for installation, and cost of installation, can make them undesirable under certain circumstances.
RADAR, which stands for “radio detection and ranging”, is the technology that uses electromagnetic radio waves to detect moving vehicles and measure their speeds. A radar gun can be used to aim a beam of radio waves of a specific frequency at a vehicle and receive the returned signal as it is reflected off the target vehicle. The relative speed of the source of the emitted electromagnetic signal and the target vehicle causes the frequency of the returned signal to shift, which is known as the Doppler Effect. The magnitude of the Doppler shift is proportional to the relative speed between the source and the target vehicle. Therefore, the speed of a target vehicle can be inferred by comparing the frequency of the reflected signal to that of the emitted signal. Although radar is accurate in determining the speeds of relatively isolated vehicles, it is less effective at differentiating various vehicles in a cluttered environment, due to the relatively large beam size. In addition, radio signals can be easily detected by radar detectors. Therefore, drivers can use radar detectors to detect the presence of radar speed measuring devices and slow down temporarily to avoid being ticketed.
An alternative technology, LiDAR, uses pulsed laser light instead of radio waves to measure vehicle speeds. A LiDAR device can aim a pulsed narrow beam of light at a target vehicle and measure the time it takes to receive the reflected signal. Knowing the speed of light, the range of the target vehicle can be calculated. If the laser pulse is applied continuously and the target vehicle is moving, the range will change with time and the difference in time it takes the transmitted signal to strike the target and return can be used to measure the speed of the target vehicle. The laser beam is much narrower compared to the beam width of radar systems. Therefore, LiDAR can pinpoint to a particular target and is also more difficult to detect.
It is crucial in speed law enforcement that vehicle speeds obtained by speed measuring devices are accurate and credible. It is typically recommended that speed measuring devices undergo certain rigorous testing and certification procedures to ensure their accuracy and compliance to defined specifications. In addition, personnel that operate speed measuring devices are usually required to receive proper training on the correct operation of the devices. However, even with all the precautious procedures to ensure proper functioning of speed measuring devices in place, measurement errors are unavoidable due to the intrinsic limitations associated with each speed measuring method.